I really don't agree. When software/UIs were simpler, the buttons screaming loudly "I AM A BUTTON!" worked. Now it quickly gets way too busy. It was important when almost everybody was a computer novice and couldn't detect subtle hints and conventions about what is a button and what isn't. Now? Not as much.
Look at screenshots of older iTunes vs Apple Music. I can feel absolutely no difference in how fast my eyes finds play/pause buttons. I don't need borders around the icons when there's not a whole bunch of skeuomorphic noise around them, such that there being any visual complexity at all (icon itself) instantly grabs attention. I really can't see a single thing I think the old iTunes interface does better for common use cases. Not that the old iTunes was bad per se, just a different UI for a different time.
I agree that often UI designers go too far, but I think you can see a bit of back and forth going on, trying to tune into the perfect balance of simplicity and clear signaling.
Obviously, everyone is different. So it's not going to be perfect for everybody no matter which way they do it.